Photoelectric conversion device

ABSTRACT

It is an object of the present invention to improve photoelectric conversion efficiency in a photoelectric conversion device. The photoelectric conversion device  11  according to the present invention uses a polycrystalline semiconductor layer including a plurality of semiconductor particles  3   a  coupled together as a light-absorbing layer  3 , each of the semiconductor particles  3   a  including a group I-III-VI compound, each of the semiconductor particles  3   a  having a higher composition ratio P I  of a group I-B element to a group III-B element in a surface portion thereof than that in a central portion thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/007,897 filed on 26 Sep. 2013, which claims the benefit of PCTApplication No. PCT/JP2012/056649 filed on 15 Mar. 2012, which claimsthe benefit of Japanese Application No. 2011-097166, filed on 25 Apr.2011. The content of each of the above applications is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a photoelectric conversion deviceincluding a group I-III-VI compound semiconductor.

BACKGROUND ART

Some photoelectric conversion devices for use in solar photovoltaicpower generation and the like include light-absorbing layers made ofchalcopyrite-based group I-III-VI compound semiconductors such as CISand CIGS. Such a photoelectric conversion device is disclosed, forexample, in Japanese Patent Application Laid-Open No. 8-330614 (1996)(which is referred to hereinafter as Patent Literature 1).

The group I-III-VI compound semiconductors, which are high in opticalabsorption coefficient, are suitable for the reduction in thickness ofphotoelectric conversion devices, the increase in area thereof and thesuppression of manufacturing costs thereof. Research and development ofnext-generation solar cells using the group I-III-VI compoundsemiconductors has been promoted.

A photoelectric conversion device including such a group I-III-VIcompound semiconductor is configured to include a plurality ofphotoelectric conversion cells arranged two-dimensionally injuxtaposition. Each of the photoelectric conversion cells includes alower electrode such as a metal electrode, a semiconductor layerincluding a light-absorbing layer, a buffer layer and the like, and anupper electrode such as a transparent electrode and a metal electrode,which are stacked in the order named on a substrate made of glass andthe like. Adjacent ones of the photoelectric conversion cells areelectrically connected in series with each other by electricallyconnecting the upper electrode of one of the adjacent photoelectricconversion cells and the lower electrode of the other of the adjacentphotoelectric conversion cells with a connection conductor.

SUMMARY OF INVENTION

An improvement in photoelectric conversion efficiency is constantlyrequired for a photoelectric conversion device including a groupI-III-VI compound semiconductor. It is therefore an object of thepresent invention to improve photoelectric conversion efficiency in aphotoelectric conversion device.

A photoelectric conversion device according to one embodiment of thepresent invention uses a polycrystalline semiconductor layer including aplurality of semiconductor particles coupled together as alight-absorbing layer, each of the semiconductor particles including agroup I-III-VI compound, each of the semiconductor particles having ahigher composition ratio P_(I) of a group I-B element to a group III-Belement in a surface portion thereof than that in a central portionthereof.

According to the aforementioned embodiment of the present invention, theprovision of the photoelectric conversion device having highphotoelectric conversion efficiency is achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a photoelectric conversion device as seenobliquely from above according to one embodiment of the presentinvention.

FIG. 2 is a schematic view showing a cross section of the photoelectricconversion device of FIG. 1.

FIG. 3 is a schematic view showing the presence of semiconductorparticles in a light-absorbing layer.

FIG. 4 is an enlarged view of the light-absorbing layer of FIG. 3.

FIG. 5 is a sectional view schematically showing a photoelectricconversion device in the course of the manufacture thereof.

FIG. 6 is a sectional view schematically showing the photoelectricconversion device in the course of the manufacture thereof.

FIG. 7 is a sectional view schematically showing the photoelectricconversion device in the course of the manufacture thereof.

FIG. 8 is a sectional view schematically showing the photoelectricconversion device in the course of the manufacture thereof.

FIG. 9 is a sectional view schematically showing the photoelectricconversion device in the course of the manufacture thereof.

FIG. 10 is a graph showing distributions of elemental composition ratiosin a semiconductor particle in the light-absorbing layer.

FIG. 11 is a graph showing elemental composition ratios in asemiconductor particle and photoelectric conversion efficiency.

FIG. 12 is a graph showing elemental composition ratios in asemiconductor particle and photoelectric conversion efficiency.

FIG. 13 is a schematic view showing a cross section of the photoelectricconversion device according to another embodiment of the presentinvention.

EMBODIMENT FOR CARRYING OUT THE INVENTION

A photoelectric conversion device according to one embodiment of thepresent invention will be described with reference to the drawings. Inthe drawings, parts comprising the same components and having samefunctions are designated by the same reference signs and duplicatedexplanations are omitted in the following explanations. The drawings areshown schematically, and the size, positional relationship and the likeof the various structures are not precisely shown in the figures.

<(1) Configuration of Photoelectric Conversion Device>

FIG. 1 is a perspective view showing a configuration of a photoelectricconversion device 11. FIG. 2 is an XZ sectional view of thephotoelectric conversion device 11 of FIG. 1. A right-handed XYZcoordinate system in which the direction of arrangement of photoelectricconversion cells 10 (left-hand and right-hand directions as seen inFIG. 1) is defined as the direction of an X axis is added to FIGS. 1 to3.

The photoelectric conversion device 11 is configured to include theplurality of photoelectric conversion cells 10 arranged in juxtapositionon a substrate 1. For purposes of illustration, only two photoelectricconversion cells 10 are shown in FIG. 1. In the actual photoelectricconversion device 11, a large number of photoelectric conversion cells10 are arranged in a plane (two-dimensionally) in the direction of the Xaxis in the figures and/or in the direction of a Y axis in the figures.

Each of the photoelectric conversion cells 10 mainly includes a lowerelectrode layer 2, a light-absorbing layer (hereinafter referred to alsoas a first semiconductor layer) 3, a second semiconductor layer 4, anupper electrode layer 5, and collecting electrodes 7. A main surface ofthe photoelectric conversion device 11 on which the upper electrodelayer 5 and the collecting electrodes 7 are provided serves as a lightreceiving surface. Three types of groove portions referred to as firstto third groove portions P1, P2 and P3 are also provided in thephotoelectric conversion device 11.

The substrate 1 supports the plurality of photoelectric conversion cells10, and made of a material selected from the group comprising of glass,ceramics, resins, metal and the like, for example. The substrate 1 usedherein is made of soda-lime glass having a thickness on the order of 1to 3 mm.

The lower electrode layer 2 is a conductive layer provided on one mainsurface of the substrate 1, and is made of a metal selected from thegroup comprising of molybdenum (Mo), aluminum (Al), titanium (Ti),tantalum (Ta), gold (Au) and the like or a laminated structure of thesemetals. The lower electrode layer 2 has a thickness on the order of 0.2to 1 μm, and is formed by a known thin film formation method such as asputtering method or a vapor deposition method, for example.

The first semiconductor layer 3 serving as the light-absorbing layer isa semiconductor layer provided on a main surface (referred to also asone main surface) on the positive Z side of the lower electrode layer 2and having a first conductivity type (herein, a p type), and has athickness on the order of 1 to 3 μm. The first semiconductor layer 3 isa polycrystalline semiconductor layer formed by coupling a plurality ofsemiconductor particles mainly made of a semiconductor ofchalcopyrite-based group I-III-VI compound (referred to also as a groupI-III-VI compound semiconductor).

From the viewpoint of improving the photoelectric conversion efficiency,the semiconductor particles forming the first semiconductor layer 3 mayhave an average particle diameter of not less than 200 nm. From theviewpoint of improving the adhesion to the lower electrode layer 2, thesemiconductor particles forming the first semiconductor layer 3 may havean average particle diameter of not more than 1000 nm. The averageparticle diameter of the semiconductor particles is measured in a mannerto be described below. First, images (referred to also as sectionalimages) of a cross section of the first semiconductor layer 3 at tenarbitrary balanced locations are acquired by photographing with ascanning electron microscope (SEM). Next, a transparent film is overlaidon each of the sectional images, and grain boundaries are traced overthe transparent film with a pen. At this time, a line (referred to alsoas a scale bar) indicating a predetermined distance (for example, 1 μm)displayed near a corner of each sectional image is also traced with apen. A scanner is used to read the transparent film on which the grainboundaries and the scale bar are written with the pen, thereby acquiringimage data. Predetermined image processing software is used to calculatethe areas of the respective semiconductor particles from theaforementioned image data. Particle diameters in the case where thesemiconductor particles are regarded as being spherical are calculatedfrom the calculated areas. The average particle diameter is calculatedfrom the average value of the particle diameters of the plurality ofsemiconductor particles captured by the ten sectional images.

The group I-III-VI compound is a compound of a group I-B element (in thepresent description, group names are described according to the oldIUPAC system; a group I-B element is also referred to as a group 11element according to the new IUPAC system), a group III-B element (alsoreferred to as a group 13 element), and a group VI-B element (alsoreferred to as a group 16 element). Examples of the group compound usedherein include CuInSe₂ (copper indium diselenide; referred to also asCIS), Cu(In,Ga)Se₂ (copper indium gallium diselenide; referred to alsoas CIGS), and Cu(In,Ga)(Se,S)₂ (copper indium galliumdi(selenide/sulfide); referred to also as CIGSS). It should be notedthat the first semiconductor layer 3 may be formed by a thin film ofmultinary compound semiconductor such as copper indium galliumdiselenide having a copper indium gallium di(selenide/sulfide) layer inthe form of a thin film as a surface layer. It should be noted that thelight-absorbing layer 3 used herein is made of CIGS.

FIG. 3 is a schematic view showing a configuration, with attentionfocused on the lower electrode layer 2, the first semiconductor layer 3and the second semiconductor layer 4. The first semiconductor layer 3has a polycrystalline structure including a plurality of semiconductorparticles 3 a coupled together. FIG. 4 is a view showing this firstsemiconductor layer 3 further enlarged.

Each of the semiconductor particles 3 a constituting the firstsemiconductor layer 3 includes a group I-III-VI compound. In each of thesemiconductor particles 3 a, the composition ratio P_(I) of the groupI-B element to the group III-B element is higher in a surface portion ofeach semiconductor particle 3 a than in a central portion thereof. Sucha configuration decreases the resistance value of the surface portion ofeach semiconductor particle 3 a to suppress the recombination ofcarriers near the grain boundaries. As a result, the photoelectricconversion efficiency of the photoelectric conversion device 11 isimproved.

In each of the semiconductor particles 3 a, the composition ratio P_(VI)of the group VI-B element to the group III-B element may be higher inthe surface portion of each semiconductor particle 3 a than in thecentral portion thereof. Such a configuration decreases group VI-Belement defects in the surface portion of each semiconductor particle 3a to suppress the recombination of carriers near the grain boundaries.The higher proportions of the group I-B element and the group VI-Belement in the surface portion near the grain boundaries increases theenergy position of the conduction band of the surface portion to shortenthe residence time of carriers, thereby suppressing the recombination ofthe carriers near the grain boundaries. Because of these facts, thephotoelectric conversion efficiency of the photoelectric conversiondevice 11 is further improved.

It should be noted that the surface portion of each semiconductorparticle 3 a refers to a region including the grain boundaries of thesemiconductor particles 3 a and extending from the grain boundarysurface to one tenth of the diameter of each semiconductor particle 3 a.The central portion of each semiconductor particle 3 a refers to aregion lying inside the aforementioned surface portion. From theviewpoint of providing good conductivity of carriers to further improvethe photoelectric conversion efficiency, the composition ratio P_(I) ofthe group I-B element to the group III-B element and the compositionratio P_(VI) of the group VI-B element to the group III-B element ineach semiconductor particle 3 a may increase gradually toward thesurface of each semiconductor particle 3 a. For example, whencomparisons are made between points 3 a 1 to 3 a 5 in a semiconductorparticle 3 a in FIG. 4, P_(I) and P_(VI) may increase gradually from thepoint 3 a 5 toward the point 3 a 1.

It should be noted that the aforementioned composition ratio P_(I) andthe composition ratio P_(VI) are measured in a manner to be describedbelow. First, a cross section of the first semiconductor layer 3 isobserved with a transmission electron microscope (TEM), and an EDSanalysis is performed at a desired point in a semiconductor particle 3a. The ratio M_(I) of the number of atoms of the group I-B element, theratio M_(III) of the number of atoms of the group III-B element, and theratio M_(VI) of the number of atoms of the group VI-B element at thatpoint are measured by the EDS analysis. From these measurement results,the composition ratio P_(I) of the group I-B element to the group III-Belement is given by M_(I)/M_(III), and the composition ratio P_(VI) ofthe of the group VI-B element to the group III-B element is given byM_(I)/M_(III).

FIG. 10 shows an example of distributions of the composition ratio P_(I)and the composition ratio P_(VI) at a point immediately over a grainboundary and points displaced at intervals of 50 nm from the grainboundary toward the center for a semiconductor particle 3 a having aparticle diameter of 500 nm. In FIG. 10, the composition ratio P_(I) atthe point in the central portion (200 nm apart from the grain boundary)of the semiconductor particle 3 a is 1.2, whereas the composition ratioP_(I) at the point in the surface portion (0 nm apart from the grainboundary, i.e. immediately over the grain boundary) is 2.4 which isgreater than the former. Also, the composition ratio P_(VI) at the pointin the central portion (200 nm apart from the grain boundary) of thesemiconductor particle 3 a is 2.1, whereas the composition ratio P_(VI)at the point in the surface portion (0 nm apart from the grain boundary)is 2.5 which is greater than the former.

The second semiconductor layer 4 is a semiconductor layer provided onone main surface of the first semiconductor layer 3. The secondsemiconductor layer 4 has a conductivity type (herein, an n type)different from that of the first semiconductor layer 3. A junctionbetween the first semiconductor layer 3 and the second semiconductorlayer 4 provides good charge separation of positive and negativecarriers generated by the photoelectric conversion in the firstsemiconductor layer 3. It should be noted that semiconductors differentin conductivity type refer to semiconductors different in conductivecarriers. When the conductivity type of the first semiconductor layer 3is the p type as mentioned above, the conductivity type of the secondsemiconductor layer 4 may be an i type, rather than the n type. Analternative configuration can be such that the conductivity type of thefirst semiconductor layer 3 is the n or i type whereas the conductivitytype of the second semiconductor layer 4 is the p type.

The second semiconductor layer 4 is made of a compound semiconductorsuch as cadmium sulfide (CdS), indium sulfide (In₂S₃), zinc sulfide(ZnS), zinc oxide (ZnO), indium selenide (In₂Se₃), In(OH,S), (Zn,In)(Se,OH), and (Zn,Mg)O. From the viewpoint of reducing losses inelectrical current, the second semiconductor layer 4 may have aresistivity of not less than 1 Ω·cm. It should be noted that the secondsemiconductor layer 4 is formed by a chemical bath deposition (CBD)method, for example.

The second semiconductor layer 4 has a thickness extending in adirection normal to the one main surface of the first semiconductorlayer 3. This thickness is in the range of 10 to 200 nm, and may be 100to 200 nm from the viewpoint of suppressing damages during theproduction of the upper electrode layer 5 on the second semiconductorlayer 4 by a sputtering method and the like.

The upper electrode layer 5 is a transparent conductive film provided onthe second semiconductor layer 4 and having a conductivity type that isthe n type. The upper electrode layer 5 is an electrode for drawingelectrical charges generated in the first semiconductor layer 3. Theupper electrode layer 5 is made of a material having a resistivity lowerthan that of the second semiconductor layer 4. The upper electrode layer5 includes what is called a window layer. When a transparent conductivefilm is provided in addition to the window layer, these may be regardedintegrally as the upper electrode layer 5.

The upper electrode layer 5 mainly include a transparent low-resistancematerial having a wide band gap. Examples of such a material used hereininclude metal oxide semiconductors such as ZnO, In₂O₃ and SnO₂. Thesemetal oxide semiconductors may include an element selected from thegroup comprising of Al, B, Ga, In, F and the like. Specific examples ofthe metal oxide semiconductors including such elements include AZO(Aluminum Zinc Oxide), GZO (Gallium Zinc Oxide), IZO (Indium ZincOxide), ITO (Indium Tin Oxide) and FTO (Fluorine Tin Oxide).

The upper electrode layer 5 is formed to have a thickness in the rangeof 0.05 to 3.0 μm by a sputtering method, a vapor deposition method, achemical vapor deposition (CVD) method or the like. From the viewpointof drawing electrical charges from the first semiconductor layer 3 well,the upper electrode layer 5 may have a resistivity of less than 1 Ω·cmand a sheet resistance of not more than 50 Ω/□.

The second semiconductor layer 4 and the upper electrode layer 5 may bemade of a material having the property of being easily light-permeable(referred to also as a light permeability) to a wavelength range oflight which the first semiconductor layer 3 absorbs. This reduces thelowering of light absorption efficiency in the first semiconductor layer3 which is caused by the provision of the second semiconductor layer 4and the upper electrode layer 5.

The upper electrode layer 5 may have a thickness in the range of 0.05 to0.5 μm from the viewpoints of enhancing the effect of preventing lossesof light reflection and a light scattering effect and transmittingelectrical current generated by the photoelectric conversion well aswell as enhancing the light permeability. Further, the upper electrodelayer 5 and the second semiconductor layer 4 may be made approximatelyequal in absolute refractive index to each other from the viewpoint ofreducing the losses of light reflection at an interface between theupper electrode layer 5 and the second semiconductor layer 4.

The collecting electrodes 7 are disposed in spaced apart relation in thedirection of the Y axis. Each of the collecting electrodes 7 extends inthe direction of the X axis. The collecting electrodes 7 are conductiveelectrodes, and are made of metal such as silver (Ag), for example.

The collecting electrodes 7 have the function of collecting theelectrical charges generated in the first semiconductor layer 3 anddrawn in the upper electrode layer 5. The provision of the collectingelectrodes 7 achieves the reduction in the thickness of the upperelectrode layer 5.

The electrical charges collected by the collecting electrodes 7 and theupper electrode layer 5 are transmitted through a connection conductor 6provided in the second groove portion P2 to an adjacent one of thephotoelectric conversion cells 10. The connection conductor 6 is formed,for example, by part of the collecting electrodes 7 extending in thedirection of the Y axis, as shown in FIG. 2. Thus, the lower electrodelayer 2 of one of the adjacent photoelectric conversion cells 10 and thecollecting electrodes 7 of the other of the adjacent photoelectricconversion cells 10 are electrically connected in series with each otherthrough the connection conductor 6 provided in the second groove portionP2 in the photoelectric conversion device 11. The connection conductor 6is not limited to this, but may be formed by an extending part of theupper electrode layer 5.

The collecting electrodes 7 may have a width in the range of 50 to 400μm so as to minimize the reduction in the light-receiving area whichinfluences the amount of light incident on the first semiconductor layer3 while ensuring the good conductivity thereof.

<(2) Method of Manufacturing Photoelectric Conversion Device>

FIGS. 5 to 9 are sectional views schematically showing the photoelectricconversion device 11 in the course of the manufacture thereof. It shouldbe noted that each of the sectional views shown in FIGS. 5 to 9 shows aportion corresponding to a cross section shown in FIG. 2 in the courseof the manufacture thereof.

As shown in FIG. 5, the lower electrode layer 2 made of Mo and the likeis initially deposited on substantially the entire surface of thecleaned substrate 1 by using a sputtering method and the like. Then, thefirst groove portion P1 is formed which extends from a linearto-be-formed position along the Y direction at the upper surface of thelower electrode layer 2 to the upper surface of the substrate 1immediately thereunder. The first groove portion P1 may be formed by ascribing process in which grooving is performed by irradiating theto-be-formed position with laser light using a YAG laser and the likewhile scanning the to-be-formed position therewith, for example. FIG. 6is a view showing a state after the formation of the first grooveportion P1.

After the first groove portion P1 is formed, the first semiconductorlayer 3, the second semiconductor layer 4 and the upper electrode layer5 are formed in order on the lower electrode layer 2. FIG. 7 is a viewshowing a state after the formation of the first semiconductor layer 3,the second semiconductor layer 4 and the upper electrode layer 5.

The first semiconductor layer 3 is formed by a process referred to aswhat is called a coating process or a printing process. In the coatingprocess, a semiconductor forming solution including an element formingthe first semiconductor layer 3 is applied onto the lower electrodelayer 2, and drying and heat treatment are thereafter performed inorder.

The semiconductor forming solution used herein may be a solution inwhich a single source precursor is dissolved in a solvent, the singlesource precursor being configured such that a group I-B element, a groupIII-B element and a group VI-B element constituting a group I-III-VIcompound semiconductor are contained in a single molecule (See thespecification of U.S. Pat. No. 6,992,202). Also various organic solventsor water are used as the solvent used for the semiconductor formingsolution. The single source precursor used herein may have a structure,for example, represented by Chemical Formula (1):

In Chemical Formula (1), E is a chalcogen element, R is an organiccompound, R-E is a chalcogen-element-containing organic compound, L is aLewis base, A is a group I-B element, and B is a group III-B element.

The chalcogen element used herein refers to S, Se and Te among groupVI-B elements. The chalcogen-element-containing organic compound refersto an organic compound containing a chalcogen element, and is an organiccompound having a covalent bond between a carbon element and a chalcogenelement. Examples of the chalcogen-element-containing organic compoundinclude thiols, sulfides, disulfides, thiophenes, sulfoxides, sulfones,thioketones, sulfonic acids, sulfonic acid esters, sulfonic acid amides,selenols, selenides, diselenides, selenoxides, selenones, tellurols,tellurides, and ditellurides. From the viewpoint of easily producing thesemiconductor forming solution, the chalcogen-element-containing organiccompound used for the single source precursor may be thiols, sulfides,disulfides, selenols, selenides, diselenides, tellurols, tellurides,ditellurides or the like which are high in coordination power to metal.

The Lewis base refers to a compound including an unshared electron pair.Examples of the Lewis base used herein include organic compoundscomprising a functional group containing a group V-B element (alsoreferred to as a group 15 element) including an unshared electron pairand a functional group containing a group VI-B element including anunshared electron pair. A compound comprising an aryl group may be usedas the Lewis base from the viewpoint of enhancing the solubility in anorganic solvent to easily produce a semiconductor forming solutionhaving a high concentration. Examples of such a Lewis base used hereininclude triarylphosphines.

A specific example of the single source precursor includes a structurerepresented by Chemical Formula (2) to be described below, for example,when the chalcogen-element-containing organic compound is phenylselenol, the Lewis base is triphenylphosphine, the group I-B element isCu, and the group III-B element is In. Another specific example of asingle source precursor includes a structure represented by ChemicalFormula (3) to be described below in which Ga is used in place of In asthe group III-B element in Chemical Formula (2). When In and Ga areincluded as the group I-III-VI compound such as CIGS, a mixture of thesingle source precursor represented by Chemical Formula (2) and thesingle source precursor represented by Chemical Formula (3) may beincluded in the semiconductor forming solution. In Chemical Formula (2)and Chemical Formula (3), Ph is a phenyl group.

The semiconductor forming solution produced in the aforementioned manneris applied onto one main surface of the lower electrode layer 2 to forma film, and a heat treatment is thereafter performed on the film, sothat the first semiconductor layer 3 is formed. For example, a spincoater, screen printing, dipping, spraying, a die coater or the like isused for the application of the semiconductor forming solution.

The heat treatment of the aforementioned film includes a step (referredto hereinafter as a thermal decomposition step) in which organiccomponents in the film are thermally decomposed, and a step (referred tohereinafter as a crystallization step) in which the group I-B element,the group III-B element and the group VI-B element react chemically witheach other to generate and crystallize a group I-III-VI compound. Watervapor is contained in an atmosphere in this thermal decomposition step.Thus, the composition ratio P_(I) of the group I-B element to the groupIII-B element can be higher in the surface portion of each semiconductorparticle 3 a than in the central portion thereof.

The thermal decomposition step is performed in an inert atmospherecontaining water vapor or in a reductive atmosphere containing watervapor, and the temperature during the thermal decomposition may be 50°to 350° C., for example. An example of this inert atmosphere includes anitrogen atmosphere. Examples of the reductive atmosphere include aforming gas atmosphere and a hydrogen atmosphere. Water vapor containedin these atmospheres may be 20 to 1000 ppm in volume fraction, forexample.

The reason why the waver vapor contained in the atmosphere in thethermal decomposition step causes the difference in the compositionratio P_(I) in each semiconductor particle 3 a has not been well known,but a phenomenon to be described below is considered to take place.First, by heating the film in an atmosphere containing water vapor, thesingle source precursor in the film is decomposed to become a complex(referred to hereinafter as a group I complex) containing a group I-Belement and a complex (referred to hereinafter as a group III complex)containing a group III-B element. The group III complex is thermallydecomposed earlier to generate a solid (referred to hereinafter as agroup III solid) containing the group III-B element, whereas the group Icomplex still remains liquid in a complex state, so that the liquidgroup I complex surrounds the group III solid. As a result,concentration distributions of the group III-B element and the group I-Belement are considered to be produced.

After this thermal decomposition step, the crystallization step isperformed. The crystallization step is performed in an inert atmospherecontaining the chalcogen element or in a reductive atmosphere containingthe chalcogen element, and the heat treatment temperature may be 400° to600° C., for example. The chalcogen element can be contained in thestates of S vapor, Se vapor, H₂S, H₂Se and the like in the atmosphere.In this crystallization step, the group I-B element, the group III-Belement and the group VI-B element chemically react with each other tobecome a polycrystal of the group I-III-VI compound. At this time, it isconsidered that the concentration distributions of the group III-Belement and the group I-B element formed in the thermal decompositionstep are maintained to a certain degree to cause the difference in thecomposition ratio P_(I) in each semiconductor particle 3 a generated.

The composition of the group VI-B element in each semiconductor particle3 a may be changed by adjusting the concentration of the chalcogenelement in the atmosphere, the rate of temperature increase and the likein the early stage of the crystallization step. For example, the heattreatment temperature is increased from near room temperature to acrystallization temperature of 400° to 600° C. at a relatively slow rateof approximately 4° to 9° C./minute in the early stage of thecrystallization step. Also, the concentration of the chalcogen elementin the atmosphere during the temperature increase is made relativelyhigh in the range of 200 to 1000 ppm in partial pressure ratio. Thus,the composition ratio P_(VI) of the group VI-B element to the groupIII-B element can be higher in the surface portion of each semiconductorparticle 3 a than in the central portion thereof. This is because thepromotion of changes to chalcogen in the early stage of thecrystallization step facilitates the formation of each semiconductorparticle 3 a and the formation of a chalcopyrite structure having a highmelting point in the surface portion of each semiconductor particle 3 a.Then, the surface portion suppresses the diffusion of the chalcogenelement toward the inside. Thus, the composition gradient of the groupVI-B element is considered to be formed.

The second semiconductor layer 4 is formed by a chemical bath depositionmethod (also referred to as a CBD method). For example, cadmium acetateand thiourea are dissolved in aqueous ammonia, and the substrate 1subjected to the processes until the formation of the firstsemiconductor layer 3 is immersed in this solution, so that the secondsemiconductor layer 4 made of CdS is formed on the first semiconductorlayer 3.

The upper electrode layer 5 is a transparent conductive film includingindium oxide containing Sn (ITO) and the like as a main ingredient, forexample, and is formed by a sputtering method, an evaporation method ora CVD method.

After the formation of the first semiconductor layer 3, the secondsemiconductor layer 4 and the upper electrode layer 5, the second grooveportion P2 is formed which extends from a linear to-be-formed positionalong the Y direction at the upper surface of the upper electrode layer5 to the upper surface of the lower electrode layer 2 immediatelythereunder. The second groove portion P2 is formed, for example, byscribing several times in succession using a scriber having a scribingwidth of the order of 40 to 50 μm while displacing a pitch. Also, thesecond groove portion P2 may be formed by scribing after the tip shapeof the scriber is widened to approximately the width of the secondgroove portion P2. Alternatively, the second groove portion P2 may beformed by scribing once to several times while two or more scribers arefixed in abutment with or in close proximity with each other. FIG. 8 isa view showing a state after the formation of the second groove portionP2. The second groove portion P2 is formed in a position slightlydisplaced in an X direction (in the figure, the positive X direction)from the first groove portion P1.

After the formation of the second groove portion P2, the collectingelectrodes 7 and the connection conductor 6 are formed. The collectingelectrodes 7 and the connection conductor 6 are formed, for example, ina manner to be described below. First, a paste (also referred to as aconductive paste) having conductivity and configured such that powder ofmetal such as Ag is dispersed in a resin binder is printed so as to drawa desired pattern. Then, the paste is dried to solidify. The solidifiedstate includes both a solid state after melting in the case where thebinder used for the conductive paste is a thermoplastic resin, and astate after hardening in the case where the binder is a hardening resinsuch as a thermosetting resin and a photo-curable resin. FIG. 9 is aview showing a state after the formation of the collecting electrodes 7and the connection conductor 6.

After the formation of the collecting electrodes 7 and the connectionconductor 6, the third groove portion P3 is formed which extends from alinear to-be-formed position at the upper surface of the upper electrodelayer 5 to the upper surface of the lower electrode layer 2 immediatelythereunder. It is preferable that the width of the third groove portionP3 is on the order of 40 to 1000 μm, for example. It is also preferablethat the third groove portion P3 is formed by mechanical scribing in amanner similar to that for the second groove portion P2. In this manner,the third groove portion P3 is formed. Thus, the photoelectricconversion device 11 shown in FIG. 1 and FIG. 2 is manufactured.

<Configuration of Photoelectric Conversion Device According to AnotherEmbodiment>

The present invention is not limited to the aforementioned embodiment,but various modifications may be made therein without departing from thespirit and scope of the present invention. For example, a plurality ofvoids may be present in the first semiconductor layer, as shown in FIG.13, in addition to the provision of the semiconductor particles in thefirst semiconductor layer, each of the semiconductor particles beingcharacterized in that the composition ratio P_(I) of the group I-Belement to the group III-B element is higher in the surface portionthereof than in the central portion thereof, as mentioned above. FIG. 13is a sectional view of a photoelectric conversion device 21 according toanother embodiment of the present invention. Same reference numerals andcharacters are used to designate parts including components and havingfunctions of the photoelectric conversion device 21 which are similar tothose of the photoelectric conversion device 11 shown in FIG. 1 and FIG.2. The photoelectric conversion device 21 differs from the photoelectricconversion device 11 in that a plurality of voids 24 are present in afirst semiconductor layer 23.

In such a configuration, when stresses are applied to the photoelectricconversion device 21, the voids 24 present in the first semiconductorlayer 23 effectively alleviate the stresses to effectively reduce cracksoccurring in the first semiconductor layer 23. On the other hand, whenthe voids 24 are present in the first semiconductor layer 23 in thismanner, defects are prone to be formed in the first semiconductor layer23 exposed to the voids 24. However, the composition ratio P_(I) of thegroup I-B element to the group III-B element is higher in the surfaceportion of each semiconductor particle constituting the firstsemiconductor layer 23 than in the central portion thereof, as mentionedabove. This suppresses the recombination of carriers near the grainboundaries of the semiconductor particles to maintain high photoelectricconversion efficiency. As a result, the photoelectric conversion device21 is high in resistance to stresses and has high photoelectricconversion efficiency.

EXAMPLE

Next, the photoelectric conversion device 11 will be described using aspecific example.

First, Steps [a] to [d] to be described next were performed in order soas to produce a semiconductor forming solution.

[a] After 10 mmol of Cu(CH₃CN)₄.PF₆ which was an organometallic complexof a group I-B element and 20 mmol of P(C₆H₅)₃ which was a Lewis basewere dissolved in 100 ml of acetonitrile, agitation was performed atroom temperature (25° C.) for five hours to prepare a first complexsolution.

[b] After 40 mmol of sodium methoxide (NaOCH₃) and 40 mmol of phenylselenol (HSeC₆H₅) which was a chalcogen-element-containing organiccompound were dissolved in 300 ml of methanol, and 6 mmol of InCl₃ and 4mmol of GaCl₃ were further dissolved therein, agitation was performed atroom temperature for five hours to prepare a second complex solution.

[c] The second complex solution prepared in Step [b] was dripped intothe first complex solution prepared in Step [a], so that a whiteprecipitate was generated. This precipitate was cleaned with methanoland was dried to obtain a precipitate including a single sourceprecursor. In this single source precursor, a single complex moleculeincludes either Cu, In and Se or Cu, Ga and Se.

[d] Pyridine which was an organic solvent was added to the precipitateincluding the single source precursor obtained in Step [c], so that asemiconductor forming solution was produced.

Next, a plurality of structures each formed by depositing a lowerelectrode layer made of Mo and the like on the surface of a substratemade of glass were prepared. The semiconductor forming solution wasapplied onto each of the lower electrode layers by a blade method toform films.

Thereafter, these films were held in nitrogen atmospheres havingdifferent water vapor concentrations (three water vapor concentrationsof 300 ppm, 50 ppm and 0 ppm in volume fraction) at 350° C. for tenminutes, so that organic components were thermally decomposed.

Next, the films subjected to this thermal decomposition wereheat-treated in an atmosphere of a gas mixture of hydrogen gas andselenium vapor gas. In this heat treatment, the temperature wasincreased from near room temperature to 550° C. for an hour, and washeld at 550° C. for an hour, so that a first semiconductor layer havinga thickness of 2 μm and made mainly of CIGS was formed.

Further, the substrates subjected to the processes until the formationof the aforementioned first semiconductor layer were immersed in asolution prepared by dissolving cadmium acetate and thiourea in aqueousammonia, so that a second semiconductor layer having a thickness of 50nm and made of CdS was formed on the first semiconductor layer. Then, anupper electrode layer made of ZnO and doped with Al was formed on thissecond semiconductor layer by a sputtering method. Thus, photoelectricconversion devices were produced.

The composition analysis of the first semiconductor layer in each of thephotoelectric conversion devices produced in this manner and themeasurement of the photoelectric conversion efficiency of eachphotoelectric conversion device were performed.

The composition analysis of the first semiconductor layer in eachphotoelectric conversion device was performed in a manner to bedescribed below. First, a cross section of the first semiconductor layerwas observed with a TEM. The composition analysis was performed by anEDS analysis at a point (a surface portion) 0 nm apart from the grainboundary toward the center of a semiconductor particle having a particlediameter of 500 nm and at a point (a central portion) 200 nm apart fromthe grain boundary toward the center thereof. The composition ratiosP_(I) and P_(VI) were calculated at each of the points. The compositionratio P_(I) in the central portion was denoted as P_(I)(centralportion), and the composition ratio P_(I) in the surface portion wasdenoted as P_(I)(surface portion). Then, the greater value ofP_(I)(surface portion)/P_(I)(central portion) is indicative of thehigher concentration of the group I-B element in the surface portion.Similarly, the composition ratio P_(VI) in the central portion wasdenoted as P_(VI)(central portion), and the composition ratio P_(VI) inthe surface portion was denoted as P_(VI)(surface portion). Then, thegreater value of P_(VI)(surface portion)/P_(VI)(central portion) isindicative of the higher concentration of the group VI-B element in thesurface portion.

The values of P_(I)(surface portion)/P_(I)(central portion) of samplesthermally decomposed with variations in water vapor concentration wereas follows: 2.3 for a water vapor concentration of 50 ppm; 2.0 for awater vapor concentration of 300 ppm; and 1 for a water vaporconcentration of 0 ppm. The values of P_(VI)(surfaceportion)/P_(VI)(central portion) thereof were as follows: 1.4 for awater vapor concentration of 50 ppm; 1.2 for a water vapor concentrationof 300 ppm; and 1 for a water vapor concentration of 0 ppm.

The measurement of the photoelectric conversion efficiency of eachphotoelectric conversion device was performed in a manner to bedescribed below. What is called a fixed light solar simulator was usedto measure the photoelectric conversion efficiency under the conditionsthat the irradiation intensity of light at a light-receiving surface ofeach photoelectric conversion device was 100 mW/cm² and AM (air mass)was 1.5. The photoelectric conversion efficiencies versus the values ofP_(I)(surface portion)/P_(I)(central portion) and P_(VI)(surfaceportion)/P_(VI)(central portion) obtained as a result of the compositionanalysis of the aforementioned first semiconductor layer are shown inFIGS. 11 and 12, respectively.

The result shown in FIG. 11 showed that the greater the value ofP_(I)(surface portion)/P_(I)(central portion), the higher thephotoelectric conversion efficiency. Also, the result shown in FIG. 12showed that the greater the value of P_(VI)(surfaceportion)/P_(VI)(central portion), the higher the photoelectricconversion efficiency.

REFERENCE SIGNS LIST

-   -   1 Substrate    -   2 Lower electrode layer    -   3 First semiconductor layer (light-absorbing layer)    -   4 Second semiconductor layer    -   5 Upper electrode layer    -   6 Connection conductor    -   7 Collecting electrodes    -   10 Photoelectric conversion cell    -   11 Photoelectric conversion device

The invention claimed is:
 1. A photoelectric conversion device wherein apolycrystalline semiconductor layer, including a plurality ofsemiconductor particles coupled together, is used as a light-absorbinglayer, wherein each of the semiconductor particles includes a groupI-III-VI compound, and wherein, in each of the plurality ofsemiconductor particles, a surface portion of the semiconductor particlehas a higher composition ratio PVI of a group VI-B element to a groupIII-B element than a central portion of the semiconductor particle. 2.The photoelectric conversion device according to claim 1, wherein agroup III-B element is included at a grain boundary of each of thesemiconductor particles.
 3. The photoelectric conversion deviceaccording to claim 1, wherein at least one of the plurality ofsemiconductor particles is positioned between two or more semiconductorparticles other than the at least one of the plurality of semiconductorparticles in a thickness direction of the light-absorbing layer.
 4. Thephotoelectric conversion device according to claim 3, wherein, in eachof the plurality of semiconductor particles, the surface portion of thesemiconductor particle has a higher composition ratio PI of a group I-Belement to a group III-B element than the central portion of thesemiconductor particle.
 5. The photoelectric conversion device accordingto claim 4, wherein the composition ratio PI and the composition ratioPVI in each of the semiconductor particles increase gradually toward asurface of each of the semiconductor particles.
 6. The photoelectricconversion device according to claim 1, wherein the group I-III-VIcompound includes Cu as the group I-B element, and includes Se as thegroup VI-B element.
 7. The photoelectric conversion device according toclaim 6, wherein the group I-III-VI compound includes a combination ofIn and Ga as the group III-B element.
 8. The photoelectric conversiondevice according to claim 1, wherein the light-absorbing layer includesa plurality of voids.